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A Night in the Coral Reef

Explore how a coral reef ecosystem changes over the course of a day. When the sun is up, colorful fish and other plant-eating animals are active and dominate the reef. At night, the plant-eaters take shelter and are replaced by nocturnal carnivores, some of which exhibit special adaptations that enable them to successfully hunt in complete darkness. Video segment adapted from NOVA

This video is available in both English and Spanish audio, along with corresponding closed captions.

In the life sciences, certain patterns begin to emerge when we shift our focus from studying individuals to studying collections of species and how they interact with and within their physical environment. Through careful observation of these patterns, we can enhance our understanding of the world around us.

We can observe several such patterns within the coral reef environment. For example, fishes inhabit different vertical bands along the reef. Some fish live on or near the sediment, others flit between the myriad coral formations, and still others swim in the shallow waters above the reef and in the open sea surrounding it.

Not only do reef dwellers occupy distinct zones, but they also exhibit discernible feeding habits. Some reef animals are diurnal, meaning they are active only during the day, while others are nocturnal, or active primarily at night.

As light diminishes, daytime animals seek shelter in the reef's caves and crevices—often the very same spaces vacated by their nocturnal counterparts. Included among the diurnal species are the plant-eaters (herbivores), some flesh-eaters (carnivores), and animals that feed on both plant and animal matter (omnivores).

By contrast, the animals most active on the reef at night are primarily carnivorous. Shrimp make their way to the surface to feed on plankton. Thus exposed, they become easy prey for larger predators. So too do crustaceans, mollusks, and starfish, which comb the sea floor at night for food. The reef's top nocturnal predators, the tiger shark, octopus, and eel, feed on these and other available prey.

Many species have evolutionary adaptations that function to enhance survival in a particular environment. Nocturnal reef dwellers have evolved unique characteristics that help them find their prey with little or no available light. For example, a type of nonvisual sensory system allows certain nocturnal fish to detect even the slightest disturbance in water when a potential prey swims past.

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